Seismic surveys are used in the oil and gas industry for exploring the subsurface geology of the earth. Seismic waves are generated that travel below the surface of the earth, reflect off subterranean structures and return to the surface where they are detected by sensors. Data from the sensors is then used to determine the location of various subsurface features that may trap oil, such as faults or folds in the rock.
The data from the sensors is processed to produce 3D volume data sets of seismic traces. The volumes include a seismic attribute value at specified (x, y, z) locations within a geographic space. A seismic attribute is a quantity extracted or derived from seismic data. Examples of seismic attributes can include measured time, amplitude, frequency, attenuation, or computed amplitude versus offset (“AVO”).
Seismic classification uses relationships between two volumes of input seismic attributes, for example, computed AVO intercept and gradient volumes, to create an output volume consisting of classification values. These classifications may allow the attributes in the output volume to be associated with a certain type of subsurface structure or lithology, such as sand or shale, and the type of fluid in a reservoir. Typically, the relationships may be specified by defining a polygon area in a cross plot of the two attributes. Conventional methods assign a constant value to each sample in the output classification volume for which the corresponding input pair of attribute samples maps to a cross plot location within the bounds of a particular polygon. Commercial volume classification tools may be divided into 3 groups. The first group involves assigning constant classified values. This technique is used in Well Seismic Fusion™, available from Landmark Graphics Corporation. The second group is typically limited to specific AVO attributes and fixed polygon shape templates. This technique is used by Well Seismic Fusion™ ABAVO plugins, available from ConocoPhillips, and AVO Crossplot, available from Headwave. The third group involves using a linear trend with bandwidth. This technique is used by Probe Paintbrush, available from Paradigm Corporation.
Conventionally, seismic classifications of two seismic attribute volumes involves assigning a constant value to each sample of the output classification volume that fall within the bounds of an individual polygon. This does not allow a seismic engineer to appreciate variations in the output data that may provide a more accurate understanding of the true subsurface conditions. Therefore, what is needed are methods and systems to create gradational classification values capturing the uncertainty or intensity of classification results. What is also needed are methods and systems that reveal subtle structural and stratigraphic features, lithography and fluids in the reservoir by applying more robust scaling method to overcome the existing limitations of volume classification